I'm building my first PostgreSQL database. Now it's just two tables (table A with 1:N relation to table B). Table A has a non-deferrable primary key, while table B has a deferrable primary key. When I try to add table C with a foreign key pointing to primary key of table B (another 1:N relation), PgAdmin refuses to create the table and it returns an error message:
ERROR: cannot use a deferrable unique constraint for referenced table
"table_B"
From my previous searches deferrable primary key seemed to be normal solution. Is it really possible in PgAdmin? Do I need some special trick for this to run?
I use PostgreSQL 9.1.3 and PgAdmin III 1.14.3 on 32 bit Windows XP.
I believe it's not possible. Quote from PostgreSQL docs about foreign key:
The referenced columns must be the columns of a non-deferrable unique
or primary key constraint in the referenced table.
Related
I'm using PostgreSQL.
I have a table accounts with account_id as the primary key. I also have a second table called relations with a composite primary key (follower_id, following_id). Each relation must be unique.
ALTER TABLE accounts ADD CONSTRAINT users_pk PRIMARY KEY (account_id);
ALTER TABLE relations ADD CONSTRAINT relations_pk PRIMARY KEY (follower_id, following_id);
I want to create a foreign key constraint from follower_id (relations) -> account_id (accounts), and the same with following_id.
ALTER TABLE relations ADD CONSTRAINT follower_id_fk FOREIGN KEY (follower_id) REFERENCES accounts (account_id) ON DELETE CASCADE
This foreign key is not accepted by the database. I get the following error:
ERROR: insert or update on table "relations" violates foreign key constraint "follower_id_fk"
DETAIL: Key (follower_id)=(4) is not present in table "accounts".
I understand this, because it's a composite primary key.
What I want to achieve:
When an account is deleted, I want to delete all the records where the account_id is the follower_id (ON DELETE CASCADE) AND where it is the following_id.
I could do this in my nodejs code or with a trigger function, but I don't know what will be the best performance-wise. Does anyone knows a/the best solution?
TL;DR
I am seeking clarity on this: does a FOREIGN KEY require a UNIQUE CONSTRAINT on the other side, specifically, in Postgres and, generally, in relational database systems?
Perhaps, I can test this, but I'll ask, if the UNIQUE CONSTRAINT is required by the FOREIGN KEY what would happen if I don't create it? Will the Database create one or will it throw an error?
How I got there
I had earlier on created a table with a column username on which I imposed a unique constraint. I then created another table with a column bearer_name having a FOREIGN KEY referencing the previous table's column username; the one which had a UNIQUE CONSTRAINT.
Now, I want to drop the UNIQUE CONSTRAINT on the username column from the database because I have later on created a UNIQUE INDEX on the same column and intuitively I feel that they serve the same purpose, or don't they? But the database is complaining that the UNIQUE INDEX has some dependent objects and so it can't be dropped unless I provide CASCADE as an option in order to drop even the dependent object. It's identifying the FOREIGN KEY on bearer_name column in the second table as the dependent object.
And is it possible for the FOREIGN KEY to be a point to the UNIQUE INDEX instead of the UNIQUE CONSTRAINT?
I am seeking clarity on this: does a FOREIGN KEY require a UNIQUE CONSTRAINT on the other side
No it does not require only UNIQUE CONSTRAINT. It could be PRIMARY KEY or UNIQUE INDEX.
Perhaps, I can test this, but I'll ask, if the UNIQUE CONSTRAINT is required by the FOREIGN KEY what would happen if I don't create it? Will the Database create one or will it throw an error?
CREATE TABLE tab_a(a_id INT, b_id INT);
CREATE TABLE tab_b(b_id INT);
ALTER TABLE tab_a ADD CONSTRAINT fk_tab_a_tab_b FOREIGN KEY (b_id)
REFERENCES tab_b(b_id);
ERROR: there is no unique constraint matching given keys
for referenced table "tab_b"
DBFiddle Demo
And is it possible for the FOREIGN KEY to be a point to the UNIQUE INDEX instead of the UNIQUE CONSTRAINT?
Yes, it is possible.
CREATE UNIQUE INDEX tab_b_i ON tab_b(b_id);
DBFiddle Demo2
When trying to ALTER TABLE in Postgres 9.5 to create foreign key constraint: from product_template.product_brand_id to product_brand.id
ALTER TABLE public.product_template
ADD CONSTRAINT product_template_product_brand_id_fkey
FOREIGN KEY (product_brand_id)
REFERENCES public.product_brand (id) MATCH SIMPLE
ON UPDATE NO ACTION
ON DELETE SET NULL;
Returns error
ERROR: insert or update on table "product_template" violates foreign key constraint "product_template_product_brand_id_fkey"
DETAIL: Key (product_brand_id)=(12) is not present in table "product_brand".
STATEMENT: ALTER TABLE "product_template" ADD FOREIGN KEY ("product_brand_id") REFERENCES "product_brand" ON DELETE set null
Im confused why postgres is trying to find product_brand.product_brand_id, when the fkey is from product_template.product_brand_id to product_brand.id
Any ideas?
The error message simply states that there is at least one row in the table product_template that contains the value 12 in the column product_brand_id
But there is no corresponding row in the table product_brand where the column id contains the value 12
Key (product_brand_id)=(12) relates the source column of the foreign key, not the target column.
In simple terms, the value of FOREIGN KEY(product_brand_id) provided in your ALTER statement is not present in the source (product_brand) table.
Our database currently doesn't define primary keys on any tables. All of the id columns are simply unique indexes. I'm dropping those indexes and replacing them with proper primary keys.
My problem: In Postgres 8.4.7, one table in particular changes the data type from bigint to integer when I add the primary key to the table.
I've got the following table definition:
psql=# \d events
Table "public.events"
Column | Type | Modifiers
-----------------------+--------------------------+-----------------------------------------------------
id | bigint | not null default nextval('events_id_seq'::regclass)
[more columns omitted]
Indexes:
"events_id_unique_pk" UNIQUE, btree (id)
Foreign-key constraints:
"events_clearing_event_ref_fk" FOREIGN KEY (clearing_event_id) REFERENCES events(id)
"events_event_configs_id_fk" FOREIGN KEY (event_config_id) REFERENCES event_configs(id)
"events_pdu_circuitbreaker_id_fk" FOREIGN KEY (pdu_circuitbreaker_id) REFERENCES pdu_circuitbreaker(id)
"events_pdu_id_fk" FOREIGN KEY (pdu_id) REFERENCES pdus(id) ON DELETE CASCADE
"events_pdu_outlet_id_fk" FOREIGN KEY (pdu_outlet_id) REFERENCES pdu_outlet(id)
"events_sensor_id_fk" FOREIGN KEY (sensor_id) REFERENCES sensors(id)
"events_user_id_fk" FOREIGN KEY (clearing_user_id) REFERENCES users(id)
Referenced by:
TABLE "events" CONSTRAINT "events_clearing_event_ref_fk" FOREIGN KEY (clearing_event_id) REFERENCES events(id)
TABLE "event_params" CONSTRAINT "events_params_event_id_fk" FOREIGN KEY (event_id) REFERENCES events(id) ON DELETE CASCADE
Triggers:
event_validate BEFORE INSERT OR UPDATE ON events FOR EACH ROW EXECUTE PROCEDURE event_validate()
This is what happens:
psql=# ALTER TABLE events ADD PRIMARY KEY (id);
NOTICE: ALTER TABLE / ADD PRIMARY KEY will create implicit index "events_pkey" for table "events"
ALTER TABLE
psql=# \d events
Table "public.events"
Column | Type | Modifiers
-----------------------+--------------------------+-----------------------------------------------------
id | integer | not null default nextval('events_id_seq'::regclass)
[more columns omitted]
Indexes:
"events_pkey" PRIMARY KEY, btree (id)
"events_id_unique_pk" UNIQUE, btree (id)
Foreign-key constraints:
"events_clearing_event_ref_fk" FOREIGN KEY (clearing_event_id) REFERENCES events(id)
"events_event_configs_id_fk" FOREIGN KEY (event_config_id) REFERENCES event_configs(id)
"events_pdu_circuitbreaker_id_fk" FOREIGN KEY (pdu_circuitbreaker_id) REFERENCES pdu_circuitbreaker(id)
"events_pdu_id_fk" FOREIGN KEY (pdu_id) REFERENCES pdus(id) ON DELETE CASCADE
"events_pdu_outlet_id_fk" FOREIGN KEY (pdu_outlet_id) REFERENCES pdu_outlet(id)
"events_sensor_id_fk" FOREIGN KEY (sensor_id) REFERENCES sensors(id)
"events_user_id_fk" FOREIGN KEY (clearing_user_id) REFERENCES users(id)
Referenced by:
TABLE "events" CONSTRAINT "events_clearing_event_ref_fk" FOREIGN KEY (clearing_event_id) REFERENCES events(id)
TABLE "event_params" CONSTRAINT "events_params_event_id_fk" FOREIGN KEY (event_id) REFERENCES events(id) ON DELETE CASCADE
Triggers:
event_validate BEFORE INSERT OR UPDATE ON events FOR EACH ROW EXECUTE PROCEDURE event_validate()
I considered a few workarounds, but I'd really rather know why it's happening. There are a few other tables that also use bigint, so I don't want to just hack a solution in place.
This is scripted with Liquibase, but it happens directly in the Postgres console too.
Update
Two other points:
I can create a simple table with a bigint id and a unique index on id, add the primary key, and the column type stays the same.
All tables are empty at the time execution.
Could it have something to do with the constraints?
That's pretty interesting. I can't reproduce it with version 9.1.0 (yes, I should upgrade too!). But then I don't know precisely how the original table and sequence were created.
This page seems to allude to a similar automatic change of types between SERIAL and INTEGER: http://grover.open2space.com/content/migrate-data-postgresql-and-maintain-existing-primary-key
Could it be something like creating the table using SERIAL instead of BIGSERIAL, and then forcing the type to BIGINT? Something in between the sequence and primary key manipulations might have reset it.
I wasn't able to reproduce this the next day, even after reproducing it multiple times with witnesses the first time it occurred. I'm chalking it up to gremlins.
I'm inserting a value in table A, that has a serial type as primary key. I wanna use the returned value of the query as a foreign key of table B... but I get this message:
ERROR: insert or update on table "tb_midia_pessoa" violates foreign key constraint "tb_midia_pessoa_id_pessoa_fkey" DETAIL: Key (id_pessoa)=(30) is not present in table "tb_pessoa". )
How can I make this possible without:
starting a new Transaction
droping my foreign keys constraints =O
?
Regards!
Pedro
You can make a deferrable FK, just use DEFERRABLE and maybe INITIALLY DEFERRED, that's up to you.
http://www.postgresql.org/docs/current/static/sql-createtable.html
The statement below allows non-deferrable constraints to be deferred until transaction committed. If you don't want to change FK definitions.
SET CONSTRAINTS ALL DEFERRED;